In the method of making cemented carbide tools from powdered materials, extremely fine powders are required. The particle size must be sub-micron, as opposed to the micron level typically used in making powdered metal components. This difference in particle size requires a different method for making the powdered product which in turn causes certain attendant problems. In order to refine the starting materials to such a fine particle size range, milling (such as by a ball milling operation) is required for significant periods of time. The smaller the particle becomes the greater the surface area and the greater the susceptibility to oxidation. Oxidized powder ingredients detrimentally affect the consolidation and physical characteristics of the compacted tool product. Accordingly, exposure to air must be minimized during such ball milling so that ingredients such as titanium carbide, tungsten carbide, molybdenum, nickel and cobalt will not oxidize.
It is conventional to employ organic substances which form a protective liquid to check oxidation during milling. Low-boiling liquids are applied because after grinding, these evaporate from the mixture automatically or at a somewhat increased temperature. Low-boiling hydrocarbons may include pentane, hexane, naptha, acetone and benzene, and in some cases chlorinated hydro carbons. Polyethylene glycol may be added as a milling aid, and is typically dissolved in a solvent when added to the mixture.
It is also conventional to press these fine powders into a tool configuration under ambient temperature conditions. To facilitate redistribution of the particles during pressing, to achieve as high a density as possible, a pressing aid, in the form of a lubricant, such as paraffin or polyethylene glycol (such as Carbowax, a commercial product of Union Carbide) is typically employed. Usually, the milled powder is allowed to air dry and Carbowax or paraffin is added as an oxidation-resistant displacement prior to pressing.
The pressing aid and/or milling aid is removed or eliminated from the powder by heating in a protective atmosphere whereby the aids are volatized and carried away in the flowing atmosphere. This latter step is conventionally referred to as the "dewaxing" step and will hereafter be used to mean a step whereby any organic milling or pressing ingredient is eliminated from the powder particles. Such dewaxing step may not necessarily be separate and independent from subsequent sintering operations, because in some instances it may be integrated as the initial heat up stage leading to a sintering temperature level.
A protective atmosphere is necessitated during the dewaxing step to insure that the milled powder remains unoxidized after removal of the protective organic medium. To this end, the art has employed atmosphere controls which have essentially consisted of (a) a 100% hydrogen atmosphere (which in some cases have included some minor dilution with argon or other inert gas), exemplified in U.S. Pat. Nos. 3,490,901; 3,798,009; and 3,762,919, (b) a vacuum essentially devoid of oxygen and nitrogen, exemplified in U.S. Pat. Nos. 3,816,081; 3,756,787; 3,762,919 and 3,964,878. When dewaxing is carried out under a vacuum, the temperature level normally employed is about 400.degree.-600.degree. C. depending upon the particular milling aid that is to be volatilized. When operating with a hydrogen atmosphere, the temperature level is typically around 400.degree.-800.degree. C. Although there is no known instances of utilizing a hybrid hydrogen atmosphere for purposes of dewaxing, it is known that in connection with the annealing of conventional steels, hydrogen based atmospheres have been employed which may contain small amounts of inert gases such as nitrogen but, usually not exceeding 25 %. Such a hybrid atmosphere is typically derived from employing cracked ammonia.
Not until the development of this invention has there been an appreciation that tool life might be related to the type of atmosphere employed during the dewaxing step. Heretofore, any increase in tool life was believed by the prior art to be a result of surface treatments, i.e., the deposition of thin oxide layers resulting from heating to 1600.degree. C. depicted in U.S. Pat. No. 3,615,884, or by the adjustment of the chemistry of the carbide cermet, depicted in U.S. Pat. Nos. 4,019,874 and 3,878,592. The prior art, as represented in these cited patents relating to tool life improvement as well as the group of patents relating to atmosphere control during milling and sintering, fail to appreciate the significance of a non-vacuum or non-hydrogen based atmosphere which facilitates enriching the surface of the carbide particle in such a manner to significantly improve tool life.